This invention relates generally to the structure of shoes. More specifically, this invention relates to the structure of athletic shoes. Still more particularly, this invention relates to variations in the structure of such shoes using the applicant""s prior invention of a theoretically-ideal stability plane as a basic concept. Still more particularly, this invention relates to the use of the theoretically ideal stability plane concept to provide stability in negative heel shoe soles that are less thick in the heel area than in the rest of the shoe sole. Still more particularly, this invention also relates to the use of the theoretically ideal stability plane concept to provide natural stability in flat shoe soles that have no heel lift, thereby maintaining the same thickness throughout; excessive structural rigidity being avoided with contoured stability sides abbreviated to only essential structural support elements to provide the shoe sole with natural flexibility paralleling that of the human foot.
The applicant has introduced into the art the general concept of a theoretically ideal stability plane as a structural basis for shoe designs. That concept as implemented into shoes such as street shoes and athletic shoes is presented in pending U.S. application Ser. Nos. 07/219,387, filed on Jul. 15, 1988; 07/239,667, filed on Sep. 2, 1988; 07/400,714, filed on Aug. 30, 1989; 07/416,478, filed on Oct. 3, 1989, and 07/424,509, filed Oct. 20, 1989, as well as in PCT application No. PCT/US89/03076 filed on Jul. 14, 1989. This application develops the application of the concept of the theoretically ideal stability plane to other shoe structures.
The purpose of the theoretically ideal stability plane as described in these pending applications was primarily to provide a neutral design that allows for natural foot and ankle biomechanics as close as possible to that between the foot and the ground, and to avoid the serious interference with natural foot and ankle biomechanics inherent in existing shoes.
In its most general form, the concept of the theoretically ideal stability plane is that the thickness of contoured stability sides of shoe soles, typically measured in the frontal plane, should equal the thickness of the shoe sole underneath the foot. The pending applications listed above all use figures which show that concept applied to embodiments of shoe soles with heel lifts, since that feature is standard to almost all shoes. Moreover, the variation in the sagittal plane thickness caused by the heel lifts of those embodiments is one of the primary elements in the originality of the invention.
However, the theoretically ideal stability plane concept is more general than those specific prior embodiments. It is clear that the concept would apply just as effectively to shoes with unconventional sagittal plane variations, such as negative heel shoe soles, which are less thick in the heel than the forefoot. Such shoes are not common: the only such shoe with even temporarily widespread commercial success was the Earth Shoe, which has not been produced since the mid-1970xe2x80x2s.
The lack of success of such shoes may well have been due to problems unrelated to the negative heel. For example, the sole of the Earth Shoe was constructed of a material that was so firm that there was almost no forefoot flexibility in the plane, as is normally required to accommodate the human foot""s flexibility there; in addition, the Earth Shoe sole was contoured to fit the natural shape of the wearer""s load-bearing foot sole, but the rigid sole exaggerated any inexactness of fit between the wearer and the standard shoe size.
In contrast, a properly constructed negative heel shoe sole may well have considerable value in compensating for the effect of the long term adverse effect of conventional shoes with heel lifts, such as high heel shoes. Consequently, effectively designed negative heel shoe soles could become more widespread in the future and, if so, their stability would be significantly improved by incorporating the theoretically ideal stability plane concept that is the basis of the applicant""s prior inventions.
The stability of flat shoe soles that have no heel lift, maintaining the same thickness throughout, would also be greatly improved by the application of the same theoretically ideal plane concept.
For the very simplest form of shoe sole, that of a Indian moccasin of single or double sole, the standard test of originally would obviously preclude any claims of new invention. However, that simple design is severely limited in that it is only practical with very thin soles. With sole thickness that is typical, for example, of an athletic shoe, the moccasin design would have virtually no forefoot flexibility, and would obstruct that of the foot.
The inherent problem of the moccasin design is that the U shape of the moccasin sole in the frontal plane creates a composite sagittal plane structure similar to a simple support beam designed for rigidity; the result is that any moccasin which is thick soled is consequently highly rigid in the horizontal plane.
The applicant""s prior application Ser. No. 07/239,667, filed on Sep. 2, 1988, includes an element to counteract such unnatural rigidity: abbreviation of the contoured stability sides of the shoe sole to only essential structural support and propulsion elements. The essential structural support elements are the base and lateral tuberosity of the calcaneus, the heads of the metatarsals, and the base of the fifth metatarsal. The essential propulsion element is the head of the first distal phalange.
Abbreviation of the contoured sides of the shoe sole to only essential structural elements constitutes an original approach to providing natural flexibility to the double sole moccasin design, overcoming its inherent limitation of thin soles. As a result, it is possible to construct naturally stable shoe soles that are relatively thick as is conventional to provide good cushioning, particularly for athletic and walking shoes, and those shoe soles can be natural in the fullest sense; that is, without any unnatural heel lift, which is, of course, an invention dating from the Sixteenth Century.
Consequently, a flat shoe sole with abbreviated contour sides would be the most neutral design allowing for natural foot and ankle biomechanics as close as possible to that between the foot and the ground and would avoid the serious interference with natural foot and ankle biomechanics inherent in existing shoes. Such a shoe sole would have uniform thickness in the sagittal plane, not just the frontal plane.
Accordingly, it is a general object of this invention to elaborate upon the application of the principle of the theoretically ideal stability plane to other shoe structures.
It is another general object of this invention to provide a shoe sole which applies the theoretically ideal stability plane concept to provide natural stability to negative heel shoe soles that are less thick in the heel area than in the rest of the shoe sole.
It is still another object of this invention to provide a shoe sole which applies the theoretically ideal stability plane concept to flat shoe soles that have no heel lift, maintaining the same thickness throughout; excessive structural rigidity being avoided with contoured stability sides abbreviated to only essential structural support elements to provide the shoe sole with natural flexibility paralleling that of the human foot.
It is still another object of this invention to provide a shoe sole wherein the abbreviation of essential structural support elements can also be applied to negative heel shoe soles, again to avoid excessive rigidity and to provide natural flexibility.
These and other objects of the invention will become apparent from a detailed description of the invention which follows taken with the accompanying drawings.